Disclosed are cementitious articles with hydrophobic finish. In some embodiments, the article is a waterproof gypsum panel that is surface reinforced with inorganic mineral fibers that face a flexible and hydrophobic cementitious finish possessing beneficial waterproofing properties. The waterproof gypsum panels of the invention are useful in many applications, such as, for example, tilebacker board in wet or dry areas of buildings, exterior weather barrier panel for use as exterior sheathing, and roof cover board having superior water durability and extremely low surface absorption. The flexible and hydrophobic cementitious finish of the invention can include Class C fly ash, film-forming polymer, silane compound (e.g., alkyl alkoxysilane), and other optional additives.